The barometric altimeter is the established primary standard of altitude used for vertical separation and landing of aircraft. However, in cases of low ceiling landings such as, for example, Category II and III, the overall accuracy of barometric altimeter system is generally deemed to be inadequate, from the viewpoint of safety. Vertically down-looking radar is capable of providing an accurate measure of distance from the aircraft to the ground, which distance is typically referred to as "radar altitude" or "terrain clearance." This data is presently employed as a second altitude reference for landing, since its accuracy is excellent at low altitudes right down to touchdown. The reading is not affected by the various errors inherent in barometric altitude systems.
In current practice, pilots employ barometric altitude to a level of about 1000 feet above landing altitude. Below that height the pilot refers to the radar altitude (terrain clearance) which provides the pilot with information relating to the ground profile under the approach path. When the profile is level, the radar altimeter may be considered by the pilot as his primary altitude reference with the barometric altimeter serving as a check, if these instruments have shown to be in agreement immediately prior thereto.
Up to the present time, these instruments are substantially independent of one another and no existing technique or apparatus exists for integrating these instruments into a composite display which is more meaningful in existing displays, is simpler to read and which provides novel inherent self-checking capabilities.